[Devel] [PATCH RHEL7 COMMIT] ms/unix: add ioctl to open a unix socket file with O_PATH

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-862.11.6.vz7.71.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-862.11.6.vz7.71.14
------>
commit 169701f225f91e3dd2d6592aa197b502a0279695
Author: Andrey Vagin <avagin at openvz.org>
Date:   Wed Sep 26 18:22:10 2018 +0300

    ms/unix: add ioctl to open a unix socket file with O_PATH
    
    ms commit: ba94f3088b792b16ea576a256a6030feddc87f24
    https://jira.sw.ru/browse/PSBM-88786
    
    This ioctl opens a file to which a socket is bound and
    returns a file descriptor. The caller has to have CAP_NET_ADMIN
    in the socket network namespace.
    
    Currently it is impossible to get a path and a mount point
    for a socket file. socket_diag reports address, device ID and inode
    number for unix sockets. An address can contain a relative path or
    a file may be moved somewhere. And these properties say nothing about
    a mount namespace and a mount point of a socket file.
    
    With the introduced ioctl, we can get a path by reading
    /proc/self/fd/X and get mnt_id from /proc/self/fdinfo/X.
    
    In CRIU we are going to use this ioctl to dump and restore unix socket.
    
    Here is an example how it can be used:
    
    $ strace -e socket,bind,ioctl ./test /tmp/test_sock
    socket(AF_UNIX, SOCK_STREAM, 0)         = 3
    bind(3, {sa_family=AF_UNIX, sun_path="test_sock"}, 11) = 0
    ioctl(3, SIOCUNIXFILE, 0)           = 4
    ^Z
    
    $ ss -a | grep test_sock
    u_str  LISTEN     0      1      test_sock 17798                 * 0
    
    $ ls -l /proc/760/fd/{3,4}
    lrwx------ 1 root root 64 Feb  1 09:41 3 -> 'socket:[17798]'
    l--------- 1 root root 64 Feb  1 09:41 4 -> /tmp/test_sock
    
    $ cat /proc/760/fdinfo/4
    pos:    0
    flags:  012000000
    mnt_id: 40
    
    $ cat /proc/self/mountinfo | grep "^40\s"
    40 19 0:37 / /tmp rw shared:23 - tmpfs tmpfs rw
    
    Signed-off-by: Andrei Vagin <avagin at openvz.org>
    Signed-off-by: David S. Miller <davem at davemloft.net>
    Signed-off-by: Cyrill Gorcunov <gorcunov at gmail.com>
---
 include/uapi/linux/un.h |  2 ++
 net/unix/af_unix.c      | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/include/uapi/linux/un.h b/include/uapi/linux/un.h
index 3ed3e46c1b1f..4f0ab3a548ad 100644
--- a/include/uapi/linux/un.h
+++ b/include/uapi/linux/un.h
@@ -10,4 +10,6 @@ struct sockaddr_un {
 	char sun_path[UNIX_PATH_MAX];	/* pathname */
 };
 
+#define SIOCUNIXFILE (SIOCPROTOPRIVATE + 0) /* open a socket file with O_PATH */
+
 #endif /* _LINUX_UN_H */
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 626259a4a4d5..5c77a0335039 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -116,6 +116,7 @@
 #include <linux/security.h>
 #include <linux/freezer.h>
 #include <linux/splice.h>
+#include <linux/file.h>
 
 struct hlist_head unix_socket_table[2 * UNIX_HASH_SIZE];
 EXPORT_SYMBOL_GPL(unix_socket_table);
@@ -2604,6 +2605,43 @@ long unix_outq_len(struct sock *sk)
 }
 EXPORT_SYMBOL_GPL(unix_outq_len);
 
+static int unix_open_file(struct sock *sk)
+{
+	struct path path;
+	struct file *f;
+	int fd;
+
+	if (!ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN))
+		return -EPERM;
+
+	unix_state_lock(sk);
+	path = unix_sk(sk)->path;
+	if (!path.dentry) {
+		unix_state_unlock(sk);
+		return -ENOENT;
+	}
+
+	path_get(&path);
+	unix_state_unlock(sk);
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		goto out;
+
+	f = dentry_open(&path, O_PATH, current_cred());
+	if (IS_ERR(f)) {
+		put_unused_fd(fd);
+		fd = PTR_ERR(f);
+		goto out;
+	}
+
+	fd_install(fd, f);
+out:
+	path_put(&path);
+
+	return fd;
+}
+
 static int unix_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 {
 	struct sock *sk = sock->sk;
@@ -2622,6 +2660,9 @@ static int unix_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 		else
 			err = put_user(amount, (int __user *)arg);
 		break;
+	case SIOCUNIXFILE:
+		err = unix_open_file(sk);
+		break;
 	default:
 		err = -ENOIOCTLCMD;
 		break;